1. Field of the Invention
The present invention relates to a pneumatic tire which has improved anti-hydroplaning performance, while preventing degradation of noise performance and steering stability.
2. Description of the Background Art
In general, in order to improve anti-hydroplaning performance of tires, drainage performance is enhanced by increasing groove width and groove depth of tread grooves provided on a tread portion. However, such measures degrade steering stability performance and noise performance on a dry road surface, with reduced pattern rigidity or increased groove volume.
Japanese Patent Application Publication No. 2004-262312 proposes a tire with tread patterns as shown in FIG. 8. On the tire, a pair of inner circumferential grooves b1 extending on both sides of a tire equator Co and a pair of outer circumferential grooves b2 extending on both outer sides of the inner circumferential grooves are provided in a tread portion “a”. With this, the tread portion “a” is divided into a center land portion c1, intermediate land portions c2, and shoulder land portions c3. In addition, the center land portion c1 and the intermediate land portion c2 are formed as a circumferential rib which continuously extends in a tire circumferential direction, and the shoulder land portion c3 is formed as a plurality of blocks sectioned by lug grooves d. In addition, in the intermediate land portion c2 are provided inclined grooves e which extend from the outer circumferential groove b2 to an axial inside of the tire while gradually reducing an angle θ with respect to the tire circumferential direction, and whose inner ends terminate near the inner circumferential groove b1. Angles θ at the tire axial inner and outer ends of the inclined grooves e and a pitch interval between the inclined grooves e, e are regulated.
The tread pattern described above has high land portion rigidity and shows excellent steering stability by the central land portion c1 and the intermediate land portion c2 being formed as the circumferential ribs. In particular, since each inclined grooves e smoothly inclines along a running water line in the intermediate land portion c2, they show excellent anti-hydroplaning performance. In addition, since the inclined grooves e have their ends terminate within the intermediate land portion c2, they ensure high land portion rigidity, and keep steering stability at high level. Furthermore, compressed air from the inclined grooves e is blocked from flowing into the inner circumferential groove b1, air-column resonance in the inner circumferential groove b1 is controlled and noise performance is improved.
However, even with such technologies, the anti-hydroplaning performance has not been improved sufficiently, and thus further improvement is requested.